Sea-lioning isn't nicely asking someone to elaborate on their view or asking for more specifics.
This is not sea-lioning:
Person 1: I really think X is bad because A, B C.
Person 2: I disagree, X is fine because D, E, F.
Person 1: eh... I disagree. And I think D is problematic too.
That's merely a civil discussion.
This is sea-lioning:
Person 1: I really think X is bad because A, B, and C.
Person 2: You're wrong. X is fine. Why don't you explain why its wrong hmm?
Person 1: I just said, because A B and C....
Person 2: A B and C are irrelevant; Why don't you explain to me why X is bad. You can't do it! You don't know what you're talking about!
Person 1: I'm actually an expert in X, I've worked on it for two decades. I don't understand why you don't see A, B, and C are problematic?
Person 2: I highly doubt you know anything about X. If you can't explain why X is bad perhaps its your own limited understanding of X. All you talk about is A B and C but you don't explain why X is bad!
(Person 2 isn't providing any real counter-arguments; they are just saying Person 1 is wrong, asking for additional evidence, and then rejecting Person 1's arguments/evidence without explaining why Person 1 is wrong.)
Keep in mind the stress here is on the personal opinion - and not statement of fact. For example, whether or not code has "cruft" is entirely a subjective manner. There is no objective measure of "cruft" in code. You cannot "prove" code is crufty. However, you can put a group of engineers in a room, have them examine some code, and they can establish a consensus as to whether or not code is in fact "crufty" despite there being no formal criteria to define it as such. Generally, it means "messy and difficult to maintain". If you are a core maintainer, and your job is to interact with this code on a regular basis, you're the de-facto expert on its quality (albeit with some personal biases). If you think the code you have to work on is bad, and people don't work on that code are saying you're wrong after just skimming it, it's going to drive you insane.